Phases of a 2D electron gas

26/03/2009

A lot of devices today are conceived to work with a 2D electron gas (2DEG). A typical and widespread application is a MOSFET where this gas makes a conducting channel with a neutralizing background of positive ions. A 2DEG is an essential part of any nanoscale device (see my preceding post) and we know that a lot of unexpected effects are seen when the temperature is lowered to few nK°, so very near absolute zero, where a fully quantum behavior should set in but something weird generally happens.

To understand these quite strange behaviors becomes mandatory to have an idea about what happens to a 2DEG changing its temperature. So, there are a lot of studies about. One of these lines of research relies on Montecarlo computations with a fixed number of electrons and taking a proper interaction between them. This people can then obtain a phase diagram of 2DEG and these findings are really interesting. A phase diagram of the 2DEG has a Wigner crystal phase at lower densities while , at higher densities the gas, in its ground state, behaves paramagnetically. This paramagnetic phase is unstable, lowering the density, and the gas enters a ferromagnetic phase! This is quite interesting as ferromagnetic states can produce such excitations as magnons that can make quantum behavior to lose its coherence. I have discussed this here (published on PRB) and here. For supporting these papers I have found a beautiful work of Giovanni Bachelet and his group here (published on PRL) where evidence is found for a ferromagnetic phase. Currently, Giovanni Bachelet has been elected at Italian Parliament for Partito Democratico (Democratic Party). You can find some biographical notes about him (in Italian) here.

The open question about these phases is to know how stable they are. A recent paper on PRL by Drummond and Needs, using the aforementioned Montecarlo methods, try to answer this question (see here).  The main conclusion they arrive is that the ferromagnetic phase does not appear to be stable while they do not find evidence for more exotic phases even if they cannot rule them out. Of course, they confirm all the preceding findings about the very existence of the known phases of 2DEG we mentioned  that since now are all well acquired. Some experimental hint exists for the ferromagnetic phase (see here) but this is not conclusive evidence.

This kind of research is really exciting being at the foundations of our understanding of behavior of matter in exotical physical situations. In the near future we will see how the complete picture will appear.


0.7 anomaly and the Fermi liquid

16/01/2009

Nanophysics is one of the research acitvities  full of promises for the improvement of our lives through the realization of new devices. This application of solid state physics becomes relevant when quantum mechanics comes into play in conduction phenomena. The aspect people may not be aware is that these researches produced several unexpected results. One of these is the so called 0.7 anomaly. This effect appears in the QPC or quantum point contacts. This can be seen as a waveguide for the wavefunction of the electrons. As such, the main effect is that conductance is quantized in integer multiples of an universal constant 2e^2/\hbar. qpc
Measurements on these devices are realized at very low temperature so to have quantum effects at work. The result of such measurements come out somewhat unexpected. Indeed, the quantization of conductance appeared as due but a further step occurred at 0.7\times 2e^2/\hbar and was called the 0.7 anomaly.

Theoretical physicists proposed two alternatives to explain this effect. The first one claimed that the Fermi liquid of conduction electrons was spin polarized while the second claimed that the Kondo effect was at work. Kondo effect appears in presence of magnetic impurities modifying the resistance curve of the material. In any case, both proposals have effects on the electron conductance and are able to explain the observed anomaly. The only way to achieve an understanding is then through further experimental work.

I have found a recent paper by Leonid Rokhinson at Purdue University, and Loren Pfeiffer and Kenw West both at Bell Lab producing a consistent result that proves that the conducting electrons are spin polarized (see here). I cannot expect a different result also in view of my paper about another problem in nanophysics and this is the appearence of a finite coherence time in nanowires, a rather shocking result for the community as the standard result should be an infinite coherence time (see here). Indeed, I have accomoned both effects as due to the same reason and this is the polariztion of the Fermi liquid (see here). This matter is still open and under hot debating in the nanophysics community. What I see here are the premises of a relevant new insight into condensed matter physics.


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